This invention relates to antifuses, and more particularly, to electrically-programmable integrated circuit antifuses.
Programmable fuses and antifuses are used in a variety of integrated circuit applications. For example, a programmable logic device may have logic that is customized by programming appropriate fuses or antifuses on the device. Fuses and antifuses may also be used to permanently switch redundant circuitry into place to fix reparable defects during the integrated circuit manufacturing process. Sometimes it may be desired to use fuses or antifuses to program a serial number or other special information into a circuit (coding).
Fuses and antifuses may be programmed using special laser-based systems or may programmed electrically.
Laser-programmable fuses are often used for integrated circuits that have aluminum interconnects. Polysilicon and aluminum fuses can be blown open by focusing a precisely-aligned laser beam on the appropriate fuses. Because each fuse must be serially-programmed, the programming process can be lengthy. The programming equipment used in laser-based systems may also be complex and expensive.
There are additional concerns when using laser-based fuses with the increasingly-popular copper interconnects. Because copper has a high reflectivity and low coefficient of thermal expansion, it is generally more difficult to use a laser to blow open a copper fuse than an aluminum fuse. There is a range of acceptable laser energies that can be used to properly blow a copper fuse. The laser energy must be sufficient to complete the laser cut through the copper. At the same time, the laser energy cannot be too high to avoid cracking.
Laser-based antifuses have also been proposed. With this type of arrangement, a vertical conducting link between two adjoining metal layers may be formed by application of the laser beam. As with laser-based fuses, laser antifuses only work within a certain range of laser energies. Laser-based antifuse programming equipment can also be complex and expensive.
Electrically programmable fuses are attractive because the need for laser-based programming equipment is eliminated and programming speeds can generally be increased.
Non-volatile memory fuses such as fuses based on electrically-programmable read-only memory (EPROM) technology or electrically-erasable programmable read-only memory (EEPROM) technology have been used. These fuses generally require high voltages to store charges in their floating gate memory cells. The high voltages may be supplied using external equipment or using integrated charge pump circuitry. While it is possible to scale down the programming voltages of these floating-gate cells by thinning the tunnel dielectric, the tunnel dielectric cannot be too thin or the data retention time of the cell will suffer. Moreover, it can be difficult to integrate non-volatile memory-based fuses into many logic circuit designs, because the special fabrication steps needed to produce the non-volatile memory-based fuses may add otherwise unnecessary constraints to the logic fabrication process.
Another type of electrically-programmable fuse that has been investigated uses silicided polysilicon devices. When high current is applied to these fuses, the resulting agglomeration of salicide on top of the polysilicon resistor increases its resistance. The programming of this type of fuse typically requires application of a programming current of 20 milliamps for a duration of 100 ms. The sensing circuits used with such fuses also tend to dissipate a large amount of power when the fuses are in their unprogrammed states. The programming process also may not be entirely permanent, because over time cobalt may migrate into the boundaries between silicide grains, thereby reducing resistance.
Electrically-programmed antifuses have been proposed that are programmed using insulator breakdown. A thin insulator is provided between two adjacent metal layers. When a sufficient voltage is applied to the metal layers, the insulator breaks down, thereby shorting the metal layers together. Although these antifuses can generally be programmed using lower powers than those used to program silicided polysilicon devices, the antifuses can be difficult to fabricate. In particular, deposition and etch-back steps needed to provide a thin oxide for the antifuses under the normally clear contact regions on the device may add undesirable complexity to the fabrication process.
Electrically-programmed antifuses based on floating-gate structures with two shortable polysilicon layers have also been proposed. With this arrangement, an oxide between the lower of the two polysilicon layers and a substrate can be broken down using a high voltage programming signal. These antifuses require 20 volts for programming. Moreover, because two polysilicon layers are required, these antifuses cannot be used on circuits that are fabricated using a single-layer polysilicon process.
It is therefore an object of the present invention to provide improved integrated circuit fuses or antifuses.